A storm approaches Adelaide: Autumn and winter rainfall in southern Australia has been declining.

Gary Rivett: ABC News

The latest climate report from the CSIRO and Bureau of Meteorology shows autumn and winter rain in southern Australia has declined sharply in recent decades.

The State of the Climate 2014 report found average Australian rainfall has actually increased since 1900, due to above average rainfall during northern wet seasons.

But in south-eastern Australia, there's been a 25 per cent reduction in average rainfall in April and May since the 1990s, and in the south-west of Western Australia there's been a 17 per cent decline in winter rain since the 1970s.

The forecast is for annual average rainfall to continue to decrease in southern Australia, with an increase in drought conditions.

Chief executive of the CSIRO, Megan Clark, says it's a significant trend that farmers need to take into account.

"If you look at whether it's below average, or very much below average, in the south-western part of Australia and in the south-eastern part of Australia, we are seeing between April and November, parts that are very much below average for rain during that time.

"And of course farmers will need to know the impact of this," Dr Clark said.

The report also found mean temperatures nationwide had increased by almost one degree since 1900, and days of extreme heat and high fire danger were occurring more regularly.

"We are projecting to continue to see an increase in more really hot days, and fewer really cold days," Dr Clark said.

"It's something we don't talk about very often with this record, but whilst we are seeing each decade we break the record of the decade before for really hot days, we're also seeing fewer really cold days."

Dr Clark says the CSIRO is focusing research on helping farmers adapt to changes in temperatures and rainfall, including developing new varieties of crops that need less water.

"We do work on the root systems themselves, making them more extensive, so they can use the nitrogen, use the nutrients in the soil to better effect.

"That's helped us as well with wheat varieties that now extend much further east in Western Australia then they have before."